Diffusion-limited electrodeposition of ultrathin Au films on Pt(1 1 1)
Introduction
The electrodeposition of metals is one of the most important processes in electrochemistry [1]. Basic aspects of the growth process have been investigated in model studies on the electrodeposition of ultrathin metal films and nanostructures, with coverages ranging from submonolayer amounts to a few atomic layers. In recent years modern structure sensitive in situ techniques such as scanning tunneling microscopy (STM) or atomic force microscopy (AFM) have greatly contributed to the understanding of these growth processes on the atomic scale. For instance, the initial stages of the deposition of thin magnetic films were investigated this way [2], [3], [4], [5]. Unlike gas phase metal deposition, e.g., via molecular beam epitaxy (MBE), fundamental studies of the electrodeposition of ultrathin metal films are often performed under conditions close to equilibrium, where the thickness and morphology of the resulting film are controlled by the thermodynamic properties of the respective system. A typical example are studies on the underpotential deposition (UPD) of mono- or bilayer metal films [1]. In contrast, the influence of the growth kinetics on the atomistic growth process and on the resulting atomic-scale film morphology, which has been intensively studied experimentally and theoretically for MBE growth (see [6], [7] and references therein), has attracted much less interest so far. Finally also the deposition potential has to be considered as an important additional parameter compared to gas phase deposition, which not only determines the kinetics of the ion transfer reaction, but also may affect the deposition process by changing the condition of the electrode surface. Examples for the latter are direct modifications of the properties of the deposited metal adatoms or indirect effects, e.g., caused by anion coadsorption. The lower interest in atomistic kinetic studies results from an experimental problem: In most experiments the deposition rate is varied together with the deposition potential, and therefore kinetic effects can hardly be separated from potential effects.
In the present paper we present and discuss in situ STM data on the growth behavior of ultrathin metal films electrodeposited under conditions where kinetic growth theories commonly used in UHV can apply. This was achieved by an alternative method, electrodeposition at high overpotentials limited by diffusion in the electrolyte, using Au electrodeposition on Pt(1 1 1) as an example. This procedure allows to independently control the potential and the deposition rate over a wide range of potentials and rates, which makes it possible to separate the influence of these two effects on the atomic-scale morphology of the deposit.
In diffusion-limited deposition at high overpotentials the metal ions or complexes in solution that arrive at the electrode surface are immediately discharged and redissolution back into the electrolyte is negligible. Hence, the deposition process occurs under conditions far from equilibrium, which resembles the situation found for MBE growth under ultrahigh vacuum (UHV) conditions, and one would expect kinetic growth theories to give a proper description of the growth process [8], [9]. According to these theories, the temporal evolution of the deposit morphology, including, e.g., the density of adlayer islands during the initial nucleation phase, the island shape during the subsequent lateral growth of monolayer islands or the film roughness during vertical growth, depend on the deposition rate and deposition temperature as experimental parameters and on system properties such as the binding energy of small admetal clusters and the mobility of metal adatoms on the flat terraces as well as along or across substrate or deposit steps [8], [9], [10], [11], [12], [13], [14], [15]. The metal flux in diffusion-limited electrodeposition is determined by the concentration of the metal species in solution, its (temperature-dependent) diffusion coefficient in the electrolyte, and the hydrodynamic conditions. It approaches a fixed value after an initial period where a steady-state diffusion profile evolves in front of the surface. The surface mobility of the deposited metal adatoms depends on the interface structure, which includes the presence of coadsorbed species (solvent molecules, anions), and on the surface charge. Consequently it strongly depends on the electrolyte composition and potential, as will be shown in this work.
The present report follows in situ STM studies of ultrathin Au films on Pt(1 1 1) [16] and AuPd films on Au(1 1 1) [17] formed by diffusion-limited electrodeposition from highly diluted solutions of the metal salts. Here we discuss for Au/Pt(1 1 1) in detail the nucleation and growth of these films from submonolayer up to multilayer coverages. After a brief description of the experimental setup and procedures we will first concentrate on the effect of the deposition potential on the nucleation and two dimensional (2D) growth behavior of monolayer Au islands on Pt(1 1 1) in Cl-free sulfuric acid solution, then evaluate the influence of strongly adsorbing anions by comparing with data obtained in Cl−-containing electrolyte, and finally investigate the multilayer growth behavior up to coverages of a few Au monolayers. The data demonstrate a large potential effect on the 2D nucleation and growth behavior, which is interpreted in terms of a modified surface mobility of the Au adatoms.
Section snippets
Experimental
A Pt(1 1 1) single crystal (Mateck), cut and oriented to better than 0.1°, was used as substrate. Before each deposition experiment, the surface was freshly prepared by electrochemical oxidation in 1 M HClO4 (1 min) and chemical dissolution of the oxide in 10% HCl (10 s). Subsequently, the crystal was annealed in a H2/air flame for 5 min, cooled down in a H2/N2 (2:98) stream, protected by a drop of Milli-Q water, and then transferred into an electrochemical cell. All solutions were made from Milli-Q
Electrochemical measurements
Prior to the STM measurements the electrochemical behavior of the samples was characterized by cyclic voltammetry in a separate electrochemical cell using a hanging meniscus configuration (Fig. 1). For a Au-free sample which has been transferred directly after flame annealing (Fig. 1a) the voltammogram exhibits the typical features of a clean Pt(1 1 1) electrode surface [21], whereas after deposition of 1.5 ML of Au a completely different voltammogram is found (Fig. 1b). In particular, the
Conclusions
We have demonstrated for Au electrodeposition on Pt(1 1 1), that electrodeposition under diffusion-limited conditions, from highly diluted solutions, and at high overpotentials allows to investigate metal deposition under well-defined, kinetically controlled conditions and to clearly separate effects from growth kinetics and potential effects. The resulting deposit morphologies closely resemble those obtained for gas phase deposition and can be explained by kinetic growth theories. The method can
Acknowledgments
We gratefully acknowledge financial support by the Deutsche Forschungsgemeinschaft and by the Centre National de la Recherche Scientifique as well as fellowships for F.M. and E.S. by the Alexander von Humboldt-foundation and for F.M. by the ‘Délégation Générale pour l’Armement’ of France.
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- 1
Present address: Laboratoire de Catalyse en Chimie Organique, CNRS UMR 6503, Université de Poitiers, 40, avenue du Recteur Pineau, F-86022 Poitiers, France.
- 2
Address: Laboratoire de Physique de la Matière Condensée, CNRS-École Polytechnique, F-91128 Palaiseau, France.
- 3
Present address: Laboratoire de Physique de la Matière Condensée, CNRS-École Polytechnique, F-91128 Palaiseau, France.